BUILDING BIG: Databank: Tower Bridge Longest Single Span: 200 feet Engineer(s): Sir Horace Jones, Sir John Wolfe-Barry By the end of the 19th century, the city of London had outgrown itself. Thousands of cars and pedestrians relied on a single bridge -- the London Bridge -- to travel in and out of the capital city each day. The traffic jams were unbearable. So it was with great anticipation that Londoners awaited the completion of a new bridge across the Thames, the Tower Bridge, designed by Sir Horace Jones and engineered by Sir John Wolfe-Barry. Click photo for larger image. The new bridge would have two towers that would rise 200 feet above the Thames. A pair of glass-covered walkways would stretch between the two towers for pedestrians. Steam engines would raise and lower the bascules, or movable roadways, in less than two minutes to allow boats to pass. Londoners were thrilled. Once the bridge was completed in 1894, however, the public was appalled with the results. Jones' original design was simple and had a medieval style. But Jones died in 1887, and Barry added his own artistic touch.

When the Tower Bridge opened to traffic in 1894, the journal The Builder cursed the bridge, calling it "the most monstrous and preposterous architectural sham that we have ever known." But public opinion mellowed over time, and today, the Tower Bridge is one of London's best loved landmarks. Here's how this bridge stacks up against some of the longest-spanning bridges in the world. (total length, in feet) Tower Bridge 880' Fast Facts: The Tower Bridge is the only movable bridge of the 29 bridges on the Thames River. When the bridge opened to river traffic in 1894, it was raised and lowered 1,000 times per year. Today, few ships actually travel on the Thames, so the bridge opens less than 100 times per year. In 1952, a London bus had to leap from one bascule to the other when the bridge began to rise with the bus still on it. Luckily, no one was injured.

Tower Bridge, London, by John Wolfe Barry and Horace Jones Tower Bridge, London, by John Wolfe Barry and Horace Jones John Wolfe Barry [ Victorian Web Home —> Science —> Technology —> Great Victorian Engineers —> Victorian Bridges and Aqueducts ] Excerpts from John Wolfe Barry's The Tower Bridge. A Lecture (1894), composed in 1893. As Barry explains in a brief preface, the account was prepared partly for people "well acquainted with engineering matters," but "more largely" for those who are simply interested, and who may "wish to form a general conception of its design and of the various considerations which led up to its inception and determined its mode of execution." — excerpts selected, most of the headings added, and photographs (when not otherwise noted) by Jacqueline Banerjee [Click on the images to enlarge them.] The West Side of Tower Bridge. Photograph (2013) by Ruth M. Landow. [ East side of the bridge by the same photographer.] The Substructure The work of the foundations was troublesome and tedious, owing to the isolation of the piers, and still more to the great amount of river traffic, rendering the berthing of barges difficult.

The substructure thus occupied a considerably longer time than was anticipated. (36) The Fixed Superstructure Two photographs showing details of the bridge's construction: Left: The massive joint that connects two sections of the bridge. Right: The laminated steel plates. Photographs by Ruth M. Landow. The fixed parts of the superstructure of the Tower Bridge consist of two shore spans, each of 270 feet, and of a central high level span of 230 feet. The fixed bridge is of the suspension form of construction, and the chains are carried on lofty towers on each pier and on lower towers on each abutment…. (45) The Piers The north tower with its pier The piers of the Tower Bridge are essentially different from the piers of an ordinary bridge, inasmuch as they have to contain the counterpoise and machinery of the opening span, as well as to support the towers which carry the suspension chains of the fixed spans and the overhead girders above the opening span. They are thus very complex structures... Their total depth from the roadway level to the London clay, on which they rest, is 102 feet.

(23, 25) The Footbridges The mode adopted for spanning the landward openings is by suspension chains, which in this case are stiffened. The chains are anchored in the ground at each end of the bridge, and united by horizontal ties across the central opening at a high level .... These ties are carried by two narrow bridges 10 feet in width, which are available as foot bridges when the bascule span is open for the passage of vessels. The foot bridges are 140 feet above Trinity high water, and, as their supports stand back 15 feet from the face of the piers, their clear span is 230 feet. Access is given to them by hydraulic lifts and by commodious staircases in the towers. (26-27) The Roadway [The landward crossing] is divided into 36 feet for the vehicular traffic and into two pathways each 1 2 feet wide. I may mention in passing that London Bridge is 54 feet wide between the parapets.

(53) The Span for River Traffic The stipulated dimensions of the opening span [provide], when the bridge is open for ships, a clear waterway of 200 feet in width, with a clear height throughout the 200 feet of 135 feet (which has been increased in construction to 140 feet) from Trinity high water mark. I may mention in passing that I think these dimensions constitute the largest opening span in the world. The next largest opening is, I believe, at the Newcastle bridge, where there are two separate spans of 100 feet each. (36-37) Opening the Bridge An early-twentieth-century photograph of Tower Bridge by Feist and Co., publishers of postcards. [the postcard version ]. I should mention that when the two leaves of the opening span are brought together, there will be long wedge-shaped bolts, actuated by hydraulic machinery, fixed on one leaf and shooting into the other leaf, to complete the union of the two.

All the machinery of the opening span will be worked from cabins on the piers, in which there will be levers like those in a railway signal box, so interlocked one with the other that all the proper movements must follow in the arranged order. (44) Time Factors The time required for the actual movement of the opening span from a position of rest horizontally to a position of rest vertically is estimated at about 1½ minutes. To this must be added the time necessary for stopping the road traffic and clearing the bridge, and withdrawing the bolts. This may take, perhaps, some 1½ minutes more, and we then have to add the time for the passage of a ship and the lowering of the bridge. The time of I½ minutes for opening or shutting the bridge gives a mean circumferential speed at the extremity of each leaf of 2 feet per second, which is a moderate speed for an opening bridge. (44) Left: Heraldry on the bridge. Right: The iron span. Signalling Procedures Signals will be provided by semaphores by day and signal lamps by night, to show ships whether the bridge is open or shut.

By night when the bridge is open for ships, four green lights will be shown in both directions, and when it is shut against ships four red lights will be similarly exhibited, and then lights will be interlocked with the machinery, so that wrong signals cannot be shown. By day similar intimation will be afforded by semaphore arms on the same posts as those which carry the signal lamps. During foggy weather, a gong will be used in specified ways. (44-45) The Lifts One other part of the machinery remains to be mentioned. This is that of the passenger lifts between the roadway level and the high level foot bridge. There are two lifts in each tower, consisting of a cage, 13 feet by 6 feet, and 9 feet high, raised and lowered by an ordinary hydraulic ram with chain gearing, and capable of lifting 20 to 25 passengers in about 1½ minutes, including the delays of opening and shutting the doors. As the lift will have to descend carrying a cargo of passengers before it can take a second load of ascending passengers, we may assume three minutes from one start to the next; or, as there are two lifts on each tower, 1½ minutes.

In addition to the lifts, there are ample flights of stairs in the towers. (45) The Towers (General Appearance and Materials) Views of one of the towers — Left: Roof, pinnacles, and windows (photograph by R. M. Landow; view in a different light ). Middle: View from the roadway approach. Right: Gothic window tracery on tower. When an opening bridge was first proposed there was some outcry by aesthetical people on the score of its ruining the picturesqueness of the Tower of London by hideous girder erections, and it seemed to be the universal wish that this bridge should be in harmony architecturally with the Tower.... it was originally intended that the towers should be of brickwork in a feudal style of architecture, and the bridge somewhat like the drawbridge of a Crusader's castle..... Sir Horace Jones unfortunately died in 1887, when the foundations had not made much progress.... Since the death of my coadjutor I have preserved the general architectural features of the Parliamentary sketch designs, but it will be seen that the structure as erected differs largely therefrom, both in treatment and material.....

[I]t became apparent that it would not be possible to support the weight of the bridge on towers wholly of masonry, as in the first designs, unless they were made of great size and unnecessary weight. It was, consequently, necessary that the main supports should be of iron or steel, which could, however, be surrounded by masonry, so as to retain the architectural character of the whole structure. (48) Interior of the Towers The skeleton of each tower consists of four wrought steel pillars, octagonal in plan, built up of rivetted plates. The pillars start from wide spreading bases, and extend upwards to the suspension chains, which they support. They are united by horizontal girders and many diagonal bracings.... The chains are carried on the abutments by similar but lower pillars.... Between the pillars are spaces for the public stairs and the passenger lifts, and for the quadrants of the opening span when in their upward position.

(48, 50) The Chains The main chains, which are 60 feet 6 inches apart from centre to centre, extend from the rollers on the piers to other rollers on each abutment, and support the platform of the bridge by suspension rods, extending from the bottom of the chains to the cross girders of the platform..... It may be asked why are these structures, which look like girders, called chains? They are, in fact, chains, stiffened to prevent deflection, and the object of the form is to distribute the local loads due to passing traffic, which, in the case of an ordinary suspension chain, distort the chain, continually depressing each part as the load passes, and consequently distorting the platform of the bridge. By making the chain, as it were, double, and bracing it with iron triangulations, these local deflections of the chain are avoided. (50-51) Weight The total weight of steel and iron in the Tower Bridge will amount to nearly 12,000 tons.

(53) Construction Time The time of construction, some 7 years to the present time, has seemed long, but it may be some comfort to those who are impatient, to remember that old London Bridge was 33 years in building, old Westminster Bridge 11¾ years, and new London Bridge 7½ years, and I think my hearers will have seen that the Tower Bridge is no ordinary bridge, and in no ordinary position. The structure and its machinery are full of the most elaborate and complicated work of all kinds. (64) Cost The cost of the bridge, with its approaches and including the cost of the property purchased, will be about a million sterling, and the whole of the expense will be defrayed out of the funds carefully husbanded and administered by the Bridge House Estates Committee. Londoners will thus be presented, without the charge of one penny on the rates, with a free bridge. The expense of working the bridge, which will be very considerable from the quantity of machinery comprised within it, will also be paid by the Corporation. (63) Barry's tributes to Colleagues First and most important of all, my acknowledgments are due to my partner, Mr. H. M.

Brunel, who has supervised the whole of the complicated calculations and details of the structure, and has taken a very active share in the carrying out of the work from first to last. Afterwards follow the resident engineer, Mr. Cruttwell, who has been in control of the works from their commencement; Mr. Fyson, who has had the duty of the preparation of most of the detailed working drawings and calculations of engineering matters, and Mr. Stevenson, who has acted as my architectural assistant. In connection with this subject, I cannot but express my great regret that the work was so soon after its commencement deprived of the architectural knowledge and experience of Sir Horace Jones, and that he has not lived to see the mode in which his conception of a large bascule bridge across the Thames has been realised.... In another branch of duty I have to express my thanks to the various contractors ... lastly, and in a very important degree, to the firm of Sir W. G. Armstrong , Mitchell and Co., to whom is entrusted the hydraulic machinery, which, I believe, is without rival in size and power.

(63-64) The Future The seagoing ships which pass the site of the Tower Bridge, and for which the central span would have to be opened, number on the average, about 17 daily. They pass by chiefly at or near the time of high water, and it may well be arranged that several may pass one behind the other. The number of seagoing ships proceeding above the site of the bridge does not show any tendency to growth, but, on the contrary, the volume of such traffic will rather, I think, gravitate to the docks down stream as time goes on. I am afraid that some disappointment will occasionally be felt when vehicular traffic is stopped by the opening of the bridge, but it may be hoped that no serious delays will occur either to seagoing ships or to vehicular traffic, as the periods during which the opening span will be raised, though sufficient for the accommodation of the river traffic, will not be of frequent occurrence or of long duration. The Tower Bridge will, it is thought, fairly meet all the difficulties of the case, but if the road traffic becomes of greater importance, and the sea-going river traffic grows less, I suppose the fate of the bridge will be to become a fixed bridge.

How soon this may happen no one can tell. It is able to fulfil its duties either as an opening or as a fixed bridge. (62-63) Conclusion In drawing this description of the works to a conclusion, I may be allowed to express a hope that the Tower Bridge, when finished, will be considered to be not unworthy of the Corporation of the greatest city of ancient or modern times. (64) Related Material The Art of Tower Bridge and Tower Bridge in Art Source Barry, John Wolfe. Tower Bridge. A Lecture . London: Boot, Son and Carpenter, 1894. Internet Archive. A copy signed "from the Author" (though mistakenly dated 1884) and uploaded by the University of California Libraries. Web. 26 February 2012. History of Tower Bridge | Topsail Events History of Tower Bridge Open Tower Bridge for your Birthday! Contact us now to make this special London landmark part of your celebrations. Why was Tower Bridge Built? London Bridge was originally the only crossing for the Thames but as the population of London grew, new bridges were needed. At this time the east end of London was a thriving, densely populated port being served by hundreds of boats including Thames Sailing Barges .

By the 19th century, journeys for pedestrians and vehicles were being delayed by hours and it was clear that London desperately needed a new bridge. Finally in 1876 the City of London Corporation decided the problem had to be addressed and made the decision to build a new bridge to the east of London Bridge Who Designed Tower Bridge? A huge challenge faced the City of London Corporation – how to build a bridge downstream from London Bridge without disrupting river traffic activities. A special committee formed in 1876 opened the design for the new crossing to public competition. Over 50 designs were submitted for consideration, many of which are on display at Tower Bridge Exhibition. It wasn’t until October 1884 however, that Horace Jones, the City Architect, in collaboration with John Wolfe Barry, offered the chosen design for Tower Bridge as a solution. It took 8 years and 5 major contractors to complete the new bridge with a total of 432 construction workers working on the project. How did they build Tower Bridge? Two massive piers were sunk into the river bed to support the construction and over 11,000 tons of steel provided the framework for the Towers and Walkways.

The steel was clad in Cornish granite and Portland stone and designed in a gothic style that gives the Bridge its recognizable appearance How does Tower Bridge Work? When it was built, Tower Bridge was the largest and most sophisticated bascule bridge ever completed (“bascule” comes from the French for “see-saw”). These bascules were operated by hydraulics, using steam to power the enormous pumping engines which can still be seen today in the Tower Bridge exhibition. The energy created was stored in six massive accumulators, as soon as power was required to lift the Bridge, it was always readily available. The accumulators fed the driving engines, which drove the bascules up and down. Despite the complexity of the system, the bascules only took about a minute to raise to their maximum angle of 86 degrees. Open Tower Bridge Exclusively? The bridge was built for vessels like our very own Sailing Barge Will . The sail trade sustained London for centuries with vessels such as Thames Sailing Barges coming and going with all manner or cargoes. With the high masts and rigging this entailed, the new bridge for London had to be able to accommodate this height.

The Victorian engineers and architects decided against a suspension bridge and built the Tower Bridge with rising bascules that we recognise today. Sailing Barge Will till this day can still open the bridge due to her tall mast – why not join us and open this iconic London landmark exclusively for your guests? Trust us, it’s quite something! Ask us about Tower Bridge: Tower Bridge History | Historic Bridges London Bridge History Some Interesting Facts Tower Bridge has a fascinating history, which is explored in full in Tower Bridge Exhibition. Here’s some to get your started… 1910 - the high-level Walkways, which were designed so that the public could still cross the bridge when it was raised, were closed down due to lack of use. 1912 - during an emergency, Frank McClean had to fly between the bascules and the high-level walkways in his Short biplane, to avoid an accident. 1952 - a London bus driven by Albert Gunter had to leap from one bascule to the other when the bridge began to rise with the number 78 bus still on it. 1977 - Tower Bridge was painted red, white and blue to celebrate the Queen's Silver Jubilee.

(Before that, it was painted a chocolate brown colour). 1982 - Tower Bridge opened to the public for the first time since 1910, with a permanent exhibition inside called The Tower Bridge Experience. Building the Bridge It took eight years, five major contractors and the relentless labour of 432 construction workers each day to build Tower Bridge. Two massive piers were sunk into the river bed to support the construction and over 11,000 tons of steel provided the framework for the Towers and Walkways. This framework was clad in Cornish granite and Portland stone to protect the underlying steelwork and to give the Bridge a more pleasing appearance. Tower Bridge Exhibition In 1910 the high level Walkways were closed to the public due to lack of use - people arriving on the Bridge preferred to wait at street level for the bascules to close rather than heading up the stairs carrying their heavy loads. In 1982, as part of the new Tower Bridge Exhibition, the Walkways were covered and opened once again. Visitors could once again enter the walkways and experience the amazing panoramic views.

Although Tower Bridge is now powered by oil and electricity, the original steam engines maintained by a dedicated team of technical officers remain in the Victorian Engine Rooms, their original location, for all to see. Choosing a Design A huge challenge faced the City of London Corporation - how to build a bridge downstream from London Bridge without disrupting river traffic activities. To generate ideas, the "Special Bridge or Subway Committee" was formed in 1876, and opened the design for the new crossing to public competition. Over 50 designs were submitted for consideration, some of which are on display at Tower Bridge Exhibition. It wasn't until October 1884 however, that Horace Jones, the City Architect, in collaboration with John Wolfe Barry, offered the chosen design for Tower Bridge as a solution. How it Works When it was built, Tower Bridge was the largest and most sophisticated bascule bridge ever completed ("bascule" comes from the French for "see-saw"). These bascules were operated by hydraulics, using steam to power the enormous pumping engines. The energy created was stored in six massive accumulators, as soon as power was required to lift the Bridge, it was always readily available.

The accumulators fed the driving engines, which drove the bascules up and down. Despite the complexity of the system, the bascules only took about a minute to raise to their maximum angle of 86 degrees. Today, the bascules are still operated by hydraulic power, but since 1976 they have been driven by oil and electricity rather than steam. The original pumping engines, accumulators and boilers are now exhibits within Tower Bridge Exhibition’s Engine Rooms. The exhibition is in continuous development, with the new glass floor panels being its most exciting and significant change since it originally opened. Visitors can now see London life through a glass floor, a unique viewpoint 42 metres above the River Thames. We encourage everyone to plan their visit for the chance to see the magic of a bridge lift beneath their feet! Through both displays and our knowledgeable guides, visitors can learn about key events in the Bridge's history, ranging from Royal visits to dare devil stunts and enjoy our ever popular “Great Bridges of the World” display. Get the latest news - Tower Bridge | London’s Famous Bridges | London Landmarks Step inside the Engine Rooms to find one of London's true hidden gems.

Soak in the atmosphere as authentic machinery which once lifted Tower Bridge’s 1000 tonne bascules fill the space with whirring wheels and moving pistons. These beautifully maintained coal-driven engines are sure to amaze. 1886 - Building the Tower Bridge, London - Architecture of London - Archiseek - Irish Architecture 1886 – Building the Tower Bridge, London Building the Tower Bridge Description from an article in the 1930’s in Wonders of World Engineering, published by Fleetway House The most famous example of the bascule bridge is the Tower Bridge across the River Thames in the heart of London. Engineers were able to build this type of bridge without interrupting traffic on the great commercial waterway. The problem of building a bridge over a busy river with low banks so that shipping is not obstructed is one that taxes the resource and ingenuity of the engineer. He surmounts the difficulty by resorting to the opening type of bridge, of which the main types are the drawbridge or bascule bridge, turning about a horizontal axis ; the swing bridge, turning about a vertical axis ; the rolling lift bridge and the vertical lift bridge.

One of the most famous examples of the bascule type is the Tower Bridge, which spans the River Thames just below London Bridge. It is the most distinctive of London’s bridges and its construction was a masterly engineering achievement. The building of the Tower Bridge came about because the development of cross-Thames traffic had far outstripped the capacity of the existing bridges. By the year 1870 the position had become serious, and between 1874 and 1885 some thirty petitions from various public bodies were brought before the authorities urging either the widening of London Bridge or the building of a new bridge. A two days’ census taken during August 1882 showed that the average traffic for twenty-four hours over London Bridge -which at that time was only 54 feet wide-was 22,242 vehicles and 110,525 pedestrians. A committee was appointed to consider the matter and to report upon the different plans that had been proposed. These included schemes for low-level bridges with swing openings of various kinds, and high-level bridges with inclined approaches or with lifts at either end.

There was also a proposal for a railway line to be built at the bottom of the river and to carry a traveling staging with its deck projecting above high-water level. Proposals for a subway and for large paddle wheel ferry boats were also considered. one of these schemes was approved. In 1878 Horace Jones, the City architect, put forward a proposal for a low-level bridge on the bascule principle – that is, a bridge on a level with the streets with two leaves or arms that could be raised to let ships pass up and down the river and lowered to let vehicles pass to and from across the waterway. Successful bridges of this type already existed, though on a much smaller scale, at Rotterdam and Copenhagen. “Bascule” is derived from the French word for see-saw,” and the bascule bridge is a kind of drawbridge which works on a pivot and has a heavy weight at one end to balance the greater length at the other. This was the type of bridge finally decided upon, and it has proved a great success.

The Tower Bridge is, perhaps, the most famous bascule bridge in the world, and its working from the day it was first opened to the present has been perfect, far exceeding the hopes even of its most enthusiastic advocates. An Act of Parliament empowering the Corporation of the City of London to build the bridge was passed in 1885. Horace Jones was appointed architect and was knighted, but died the same year, and Mr. (afterwards Sir) John Wolfe Barry was appointed engineer. The work was divided among eight different contractors Among them Sir John Jackson was responsible for the piers and abutments, Sir William Arrol for the steel superstructure, Sir W. G. Armstrong, Mitchell and Co., Ltd., for the hydraulic machinery and Perry and Company for the masonry superstructure. Work was started on the bridge in April 1886, the foundation stone being laid, on behalf of Queen Victoria, by the Prince of Wales, afterwards King Edward VII. The bridge was to have been finished by 1889, but difficulties arose and Parliament was twice asked to extend the time for the completion of the work.

It did so, and the bridge was eventually opened on June 30, 1894, having cost about £1,000,000 sterling to build, a remarkably small sum for such a bridge in such a position. The total length of the bridge, including the approaches, is half a mile. The roadway has a width of 35 feet and on either side of it is a footway 12.5 feet wide. The total height of the towers on the piers, measured from the level of the foundations, is 293 feet. 140 Feet Headway for Ships In building the bridge there were used about 235,000 cubic feet of Cornish granite and Portland stone, 20,000 tons of cement, 70,000 cubic yards of concrete, 31,000,000 bricks and 14,000 tons of iron and steel. The bridge is a combination of the suspension and bascule type. The width of the river between the abutments of the bridge on the north and south sides is 880 feet. This is crossed by three spans.

The two side spans, each 270 feet long, are of the suspension type. They are carried on stout chains that pass at their landward ends over abutment towers of moderate height to anchorages in the shore. At their river ends the chains pass over lofty towers which are themselves connected at an elevation of 143 feet above high water. Heavy tie bars, at the level of the connecting girders, unite the two pairs of chains so that one acts as anchorage for the other at the centre. The central span has two high-level foot ways side by side, and one low-level roadway. High-level girders carry the upper footways, which are reached by hydraulic lifts or staircases in the main towers. The roadway, or central opening span, is 200 feet long and consists of two bascules or leaves. The Tower Bridge Act laid down that when the bridge was open there should be a clear headway at high tide between the water and the high-level footways of 135 feet and a headway of 29 feet when the bridge was closed. These dimensions were exceeded in practice, the open height being 5 feet and the closed height 6 in.

greater than had been prescribed. This was above high-water level. The greatest extreme between high and low tide at Tower Bridge is 25 feet. The Act further stipulated that the piers were to be 185 feet long and 70 feet wide. There was also a clause making it compulsory to maintain at all times during the building of the bridge a clear waterway 160 feet wide. This stipulation made it impossible for the two piers to be built at the same time, because the staging would have occupied far too much of the river space. As the use of timber cofferdams was prohibited, the builders had to rely on caissons. The restricted area which they were allowed for their staging, 130 feet by 335 feet, did not permit the use of one caisson extending the full length of a pier. The builders therefore adopted a system of small caissons covering the area of the pier. By this means it was possible while building one of the piers to be working also at the shore side of the other. Had both piers proceeded simultaneously a saving of thirteen or fourteen months might have been effected.

The piers of the Tower Bridge are much more complicated structures than the piers of an ordinary bridge. In addition to supporting the towers carrying the overhead girders for the high-level footways and the suspension chains of the fixed spans, they also house the counterpoise and the machinery which operates the bascules. Triangular Caissons The caissons used for securing the foundation of the piers consisted of strong boxes of wrought iron, without either top or bottom. To secure a good foundation it was found necessary to sink them to a depth of about 21 feet into the bed of the river. There were twelve caissons for each pier. On the north and south sides of each pier was a row of four caissons, each 28 feet square, joined at either end by a pair of triangular caissons, formed approximately to the shape of the finished pier. There was a space of 2.5 feet between all the caissons, this being considered the least dimension in which men could effectively work. The caissons enclosed a rectangular space 34 feet by 124.5 feet.

The space was not excavated until the permanent work forming the outside portion of the pier had been built, in the caissons and between them, up to a height of 4 feet above high-water mark. The method adopted in building and sinking the caissons was unusual. First came the building of the caisson upon wooden supports over the site where it was to be sunk. The caisson was 19 feet in height and it was divided horizontally into two lengths. The lower portion was known as the permanent caisson and the upper portion, which was removable when the pier was completed, was called the temporary caisson. The object of this upper portion was simply to keep out water while the pier was being built. When ready the supports were removed and the permanent caisson lowered to the river bed (this had previously been levelled by divers) by means of four powerful screws attached to four lowering rods. After the caisson had reached the ground various lengths of temporary caisson were added to the permanent section, till the top of the temporary portion came above the level of high water. The joint between the permanent and the temporary caissons was made tight with india-rubber.

Divers working inside the caisson excavated first the gravel and then the upper part of the clay forming the bed of the river. As they dug away the soil, which was hauled up by a crane and taken away in barges, the caisson gradually sank until its bottom edge penetrated some 5 feet to 10 feet into the solid London clay. London clay is a firm watertight stratum, and when the desired depth had been reached by the caisson it was safe to pump out the water, which up to this time had remained in the caisson, rising and falling with the tide through the sluices in the sides. The water having been pumped out, navvies were able to get to the bottom of the caisson and to dig out the clay in the dry. Additional lengths of temporary caisson were added as the caisson sank, so that at last each caisson was a box of iron 57 feet high, in which the preparation of the foundations could be made. The caisson having been controlled from the first by the lowering rods and screws, its descent any farther than was desired was easily arrested by the rods when the bottom of the caisson was 20 feet below the bed of the river.

The clay was then excavated 7 feet deeper than the bottom of the caisson, and outwards beyond the cutting edge for a distance of 5 feet on three of the four sides of the caisson. In this way not only was the area of the foundations of the pier enlarged but, as the sideways excavation adjoined similar excavations from the next caissons, the whole foundation also was made continuous. All the permanent caissons, with the spaces between them were then completely filled with concrete, upon which the brickwork and masonry were begun in the temporary caisson and carried up to 4 feet above high water. The preparation of the foundations was a long and troublesome task because of the extent of the river traffic, which made it difficult to berth the necessary barges. On two occasions “blows” occurred which hindered the operations. When the cutting edge of one of the caissons had reached a depth of 16 feet beneath the river bed, water rushed into the caisson through a rent in the clay. The caisson had to be lowered still further to seal the opening when the water was pumped out. The second blow was due to one of the stage piles between the caissons having been driven in aslant.

As the caisson went down its cutting edge came in contact with the pile and thus loosened the clay in the immediate neighbourhood. Divers were sent down to ascertain the damage and the pile was re-driven. The full extent of these handicaps was underestimated and thus this section of the work occupied much longer than had been expected. Finally there emerged four feet above high-water mark two gigantic piers of concrete, granite and bricks able to withstand without settlement a load of 70,000 tons. From the river bed upwards the piers are faced with rough picked Cornish granite, in courses between 2 feet and 2.5 feet thick. The piers called for the excavation of 30,000 cubic yards of mud, silt and London clay. The material consumed in the piers was 25,220 cubic yards of cement, 22,400 cubic yards of bricks and 3,340 cubic yards of Cornish granite. The cost of the piers was £111,122. As soon as the piers had been finished the building of the towers began.

Stone Over Steel Because of the fine masonry work of these towers, Tower Bridge is often mistaken for a stone bridge. It is a steel bridge, however, just as much as is the Forth Bridge, and it depends entirely for its strength upon the steel columns and girders of which it is composed. As the authorities insisted that the design of the bridge should be in keeping with its surroundings, the steelwork is faced with masonry whose architectural character is made to harmonize with the general style of the Tower of London close by. The masonry is Cornish granite and Portland stone, backed with brickwork. Each of the steel towers consists of four octagonal columns, with a diameter of 5 ft. 6 in., connected at a height of 60 feet above the piers by plate girders, 6 feet deep. Across these are laid smaller girders which carry the first landing. Twenty-eight feet higher is the second landing, similarly built, and at an equal distance above that is the third landing, leading to the high-level footways Each column rests on a massive granite slab previously covered with three layers of specially prepared canvas to make the pressure even and the joint watertight.

The columns are keyed to their foundations by great bolts built into the piers. All four columns in each pier are braced diagonally to resist the wind pressure, which is calculated at a maximum of 56 lb. to the square inch, a pressure several times greater than has ever been registered in the locality. It was important that precautions should be taken to prevent any adhesion between the masonry and the steelwork of the towers. With this object the columns were covered with canvas as the masonry was built round them, and spaces were left in places where any later deformation of the steel work might bring undue weight upon the adjacent stonework. The masonry covering forms an excellent protection against extremes of temperature. All parts of the metal not accessible for painting purposes after the bridge was completed were coated thoroughly with Portland cement. Manholes were provided in the steel columns to make it possible to paint the interior whenever it became necessary. The abutments of the bridge, which were built by means of cofferdams in the usual manner and without difficulty, have similar but shorter towers. The towers finished, workmen tackled the high-level footways. These are cantilever structures, each with a suspended span. They were built out from either tower simultaneously.

The footways are cantilevers for a distance of 55 feet from either tower and suspended girders for the remaining distance of 120 feet between the cantilever ends. The building of these cantilevers attracted a great amount of attention on the part of the public, who watched their gradual approach with keen interest. Every care was taken to prevent rivets, fragments and tools from falling into the river below, to the peril of passengers in passing vessels. Intricate Suspension Chains Along the upper boom of the footway run the great ties connecting the suspension chains at their river ends. Each of the two ties is 301 feet long and is composed of eight plates 2 feet deep and 1 inch thick, ending in large eye-plates to take the pins uniting them to the suspension chains. The making of these chains was one of the most interesting and at the same time most delicate parts of the whole undertaking. Each chain is composed of two parts, or links, the shorter dipping from the top of the abutment tower to the roadway, the longer rising from the roadway to the summit of the main tower.

The links have each a lower and upper boom, connected by diagonal bracing so as to form a rigid girder. They were built in the positions they had to occupy, supported on trestles, and were not freed until they had been joined by huge steel pins to the ties crossing the central span and to those on the abutment towers. The boring of the pin holes was a matter of great delicacy and considerable difficulty. The holes in the eye-plates of ties and chains had been bored to within 0.5 in. of their final diameter before leaving the contractor’s works at Glasgow, and the finishing touches were added when the plates were in position. The labour of enlarging all the holes to their full diameter was equivalent to boring a hole with a diameter of 2 ft. 6 in. through 65 feet of solid steel. Most of this boring had to be done in somewhat awkward positions at the top of the main towers and abutments, whither it was necessary to transport engines, boilers and boring tools. The outstanding feature of the bridge is its opening span, consisting of two bascules or leaves.

Each leaf consists of four parallel girders 13.5 feet apart and about 160 feet long. When lowered the leaf projects horizontally 100 feet towards the opposite tower, spanning exactly half of the opening. The point of balance is a solid pivot, with a diameter of 1 ft. 9 in. and a length of 48 feet. It passes through the girders 50 feet from their shore ends. The pivot is keyed to the girders and rotates on roller bearings carried by eight girders crossing the piers horizontally from north to south, themselves borne on girders under their ends. v The chief difficulty attending the erection of the bascules was due to the condition that compelled the contractors to leave a clear way of 160 feet between the towers. In other circumstances the girders might have been completed before being brought into line and connected together. As it was, the engineers first built the portions on the shore side of the pivot, added a short section of the riverward steelwork and launched the incomplete girders from the main stage close to the piers into the bascule chambers.

A steel mandrel (cylindrical rod) was inserted to carry their weight while they were turned into a vertical position. The mandrel was then withdrawn to make room for the permanent pivot, which weighed 25 tons. The outer ends were added to until a point 53 feet from the pivot had been reached. Work in this direction then stopped until the raising and lowering of the leaves for purposes of adjustment had been concluded. After that the girders were completed vertically. The leaves, each of which weighs about 1,200 tons, are moved by toothed pinions, engaging with steel quadrantal racks riveted to their two outside girders. The accurate attachment of the racks was a somewhat difficult business because of the confined space in which the men had to work. To preserve the balance of the bascule it was necessary to load the shorter, or inner arm with counterpoises, consisting of 290 tons of lead and 60 tons of iron enclosed in ballast boxes at the extreme ends of the girders. The function of the raising gear is merely to overcome the inertia of the 1,200-tons leaf and the friction caused by wind pressure on the exposed surface.

In designing the hydraulic machinery allowance was made for a wind pressure of 56 lb. to the square foot. Opened and Shut in Five Minutes The source of power is a building on the east side of the southern approach, where are stationed two large water accumulators with 20-in. rams loaded to give a pressure of from 700 lb. to 800 lb. per square inch. The engines are duplicated on either pier to provide against the possibility of breakdown. The operations of opening and shutting the bridge are safeguarded by every possible means. When the leaves are brought together bolts carried on one leaf are locked by hydraulic power into sockets on the other leaf. In the event of anything going wrong with the opening and closing mechanism there would be no danger of disaster, for the leaves would be brought gently to rest in either the vertical or the horizontal position. The whole process of opening the bascules, allowing a ship to pass and bringing them down again for the resumption of road traffic takes only five minutes. Thus the large hydraulic lifts, which go to the top of the tower to the overhead footway with eighteen passengers in one minute, are rarely used.

It has been found that the interruption of traffic is so brief that pedestrians do not take the trouble to go up and over the footway, but wait for the lowering of the bascules.

Question: London's Tower Bridge is a suspension and what other type of bridge?
Answer: BASCULE